I would like to drive the 7 without the side-screens (doors), but the wind turbulence is too much at speeds over 50mph. I’d seen some various deflectors at the 7’s picnic and wanted to try making one.
Iteration 1, front deflector
For a first iteration, I went with a larger size than the standard Caterham deflector (bigger is better?). This was made from 0.1″ acrylic (plexiglass) material. I had to borrow the hinges from the side-screen. I’ll have to buy some from the Caterham on-line shop.
I used some acrylic pieces that I had, but they were too small, so I had to extend by bolting on a 2nd piece. You’ll also notice a bend, this is achieved by heating the acrylic with a propane torch, very slowly and moving constantly, until the acrylic is soft enough to bend. If you get it too hot it will bubble or even catch fire. I found that it is easy to go too far and most of my bends do not look attractive (as you will see). The soft acrylic is also susceptable to thumb & finger glove prints (you have to wear gloves as it’s really hot).
I went for a test drive, it was better than nothing, but still a lot of buffeting. I thought I needed some modification, so went back home and made some bends to make the top of the deflector more streamlined with less drag. This gave me iteration 2.
Another test drive and I did not see any real improvement. What I did notice however was that wind was coming in at head level at the side of my head and blowing my hair forwards. Lacking a wind tunnel, I used my hand as a tell-tale and moved to various positions just outside the car as I was driving to determine where the wind was coming from. It appears that wind is moving along the side of the car until it reaches the rear wheel & wing (fender), then it is forced upwards. I went back home and to the drawing board.
I did draw a picture. What I think is happening is first, wind is pushed upwards and back over the front windscreen. This creates a partial vacuum inside the driver’s cockpit. Wind from the side, moving upwards around the rear wing is then sucked into the partial vacuum of the driver’s cockpit. See below picture, blue ink arrows show the wind from the side.
Iteration 3, rear side screen.
If my hypothesis is correct, I need a partial side screen at the rear of the cockpit. So I set about making one. This would be iteration 3, see picture. I managed to use 2 of the poppers to hold the screen in place in addition to a zip-tie to the roll bar, with a piece of polystyrene foam.
Time for another test drive. Much better, I can drive at 70mph with less buffeting than at 50mph without deflectors. I’m on the right track.
Iteration 3. Broken acrylic.
Before going home, I filled up with gas and promptly broke the acrylic side-screen. I guess it was rather too delicate, made of 0.1″ acrylic sheet. I had suspected that 0.1″ was too thin and had bought a piece of 0.22″ acrylic sheet – it was $50, so I have been trying the 0.1″ thick prototypes first.
Back home, and I set to work on iteration 4. This time made out of 0.22″ acrylic. Note that I’m cutting the acrylic sheet with a bandsaw, scoring 0.22″ acrylic and snapping does not work. It’s also harder to make the bends as the whole 0.22″ has to be heated throughout before a bend will happen.
Iteration 4, view 1.
Iteration 4, view 2.
___________________In the iteration 4, you will note a curve outwards. This is to accommodate my shoulder. There is a bend at the back to follow the roll bar contour. There is also a ~15degree bend just below the lower popper so that the bottom follows the side of the car.
Test drive time, up to 80mph, there is some marginal buffeting and my hair flies around, but I’m driving with open sides!
Front and rear deflectors.